More Than One Way to Get to the Proteasome

Antizyme is best known for its role in facilitating the degradation of ornithine decarboxylase, which performs the rate-limiting step in polyamine synthesis. Overexpression of antizyme or treatment of cells with exogenous polyamines, which stimulates antizyme production, also results in inhibition of cell division. Newman et al. determined that antizyme targets cyclin D1, a key regulator of the G1 to S transition, for degradation through a mechanism that does not require ubiquitin-mediated targeting to the proteasome. Prostate cancer cells (AT2.1) exposed to polyamine or overexpressing antizyme exhibited decreased cyclin D1, without any decrease in its mRNA, and the cells arrested in G1. Polyamine stimulation of cyclin D1 degradation was inhibited by antisense oligonucleotides against antizyme. Cells expressing a mutant form of cyclin D1 that cannot be effectively conjugated to ubiquitin still showed decreased amounts of cyclin D1 after spermine treatment or antizyme overexpression. Using proteins translated in vitro, a complex between cyclin D1 and antizyme was detectable. Finally, in an in vitro assay with purified proteins, antizyme stimulated cyclin D1 degradation by the proteasome in the absence of ubiquitin and ubiquitin ligases. Thus, antizyme appears to be able to target substrates for proteasomal degradation by a pathway that does not involve ubiquitination.